The present invention relates to a microwave-excited discharge lamp apparatus which emits light by discharge under a microwave electromagnetic field.
In recent years, in accordance with demands of energy saving etc., high intensity discharge lamps have been attracting attention. The reason why is that the high intensity discharge lamps can easily provide large light output compared with fluorescent lamps which give light output through phosphors. The high intensity discharge lamps are divided into two types: an electrode type discharge lamp having electrodes, such as a metal halide lamp and a mercury lamp; and an electrodeless type discharge lamp such as a microwave-excited lamp.
In the microwave-excited lamp, a predetermined microwave electromagnetic field is formed by a magnetron or the like microwave generator, and plasma discharge caused by a microwave electromagnetic field is used as a light source. The microwave-excited lamp has a long life compared with the electrode type discharge lamp limited by deterioration of the electrodes. Furthermore, in the microwave-excited lamp, an emission spectrum of the light output does not almost deteriorate after a long period of service because of electrodeless configuration.
Moreover, since change of impedance between operating condition and extinguished condition is small, the microwave-excited discharge lamp provides far greater advantages than the electrode type discharge lamp with regard to characteristics of flashing operation, starting, and restarting. The microwave-excited discharge lamp also has a remarkable advantage over the electrode type discharge lamp in view of environmental protection. The reason why is that, as has been described in the above, the microwave-excited discharge lamp has the long life, and thereby components of the microwave-excited discharge lamp need not be exchanged for a long time. Furthermore, the microwave-excited discharge lamp gives light output having the brightness and efficacy comparable to those of the electrode type discharge lamp without use of environment-harmful mercury.
A conventional microwave-excited discharge lamp apparatus disclosed in unexamined and published Japanese patent application TOKKAI Hei 3-49102 will be described with reference to FIG. 11 specifically.
As shown in FIG. 11, the conventional microwave-excited discharge lamp apparatus comprises a microwave generator 51 for generating a microwave, a waveguide 52 for propagating the microwave generated in the microwave generator 51, a cavity 53 connected to the waveguide 52, and a microwave-excited discharge lamp (hereinafter referred to as "a lamp") 54 arranged in the cavity 53.
The microwave generator 51 includes a magnetron 51a for generating a microwave of, for example, 2450 MHz with an output of several kW, an antenna 51b for radiating the microwave generated, and a fan 51c for cooling the magnetron 51a. A high voltage power supply 55 for driving is connected to the magnetron 51a.
The waveguide 52 is configured of a metal box member having a rectangular section, for example. The antenna 51b is housed at one end of the waveguide 52, and a power feeding window 52a and an aperture 52b in the opposite relation to the power feeding window 52a are formed in the other end of the waveguide 52.
The cavity 53 is formed of a metal material such as copper and includes a substantially cylindrical member 53a with open ends and a mesh plate 53b arranged on one of the open ends of the cylindrical member 53a. The other open end of the cylindrical member 53a is mounted on the surface of the waveguide 52 in such a manner as to surround the power feeding window 52a of the waveguide 52.
The internal space of the cavity 53 forms a cavity resonator for accumulating microwave energy. In the case that the microwave is radiated from the power feeding window 52a, a predetermined microwave electromagnetic field is formed in the internal space of the cavity 53. Also, the light generated in the lamp 54 by the plasma discharge is emitted outside as a light output through the mesh plate 53b. A reflector (not shown) for reflecting visible light is arranged on the inner wall of the cavity 53 in order to retrieve the light output efficiently in a single direction.
The lamp 54 is formed of translucent quartz glass or the like in a substantially spherical or elongate cylindrical form, and is arranged in the internal space of the cavity 53 by a supporting rod 56 of quartz glass. The lamp 54 hermetically contains therein a rare gas such as argon, a small amount of mercury and a metal halide such as thallium iodide providing a luminous material. The internal pressure of the lamp 54 in an extinguished condition is regulated at about 100 to 200 Torr in order to easily perform a starting operation, i.e. a starting of the below-mentioned plasma discharge of the rare gas. The supporting rod 56 is connected to a motor 57 for rotating the lamp 54 through the power feeding window 52a and the aperture 52b of the waveguide 52 and further through a connecting jig 57a. The rotation of the motor 57 stabilizes the plasma discharge in the lamp 54 while at the same time cooling the lamp 54.
A pair of nozzles 58 are arranged in the vicinity of the lamp 54 for injecting a cool air flow supplied from a compressor not shown. As a result, the lamp 54 is sufficiently cooled, thereby to prevent thermal degeneration of the lamp 54 caused by the plasma discharge. It is known that the pair of the nozzles 58 are provided corresponding to the size of the lamp 54 and the output of the magnetron 51a, and are included in the conventional microwave discharge apparatus. Specifically, in the case that the output of the magnetron 51a is not less than several kW and the spherical lamp 54 is not more than 8 mm in diameter, for example, the pair of the nozzles 58 is arranged in the vicinity of the lamp 54.
In the case that the output of the magnetron 51a is small and the lamp 54 is large in size, in contrast, the lamp 54 is sufficiently cooled by the rotation of the motor 57, so that the thermal degeneration of the lamp 54 by the plasma discharge is suppressed to some degree. In such a case, therefore, the nozzles 58 are not generally included in the conventional microwave-excited discharge lamp apparatus. Furthermore, another conventional microwave-excited discharge apparatus with a small output of the magnetron 51a and a large lamp 54 has been proposed, in which the cooling air (cooled air) from the fan 51c, after being used for cooling the magnetron 51a, is blown to the lamp 54 instead of using the motor 57. Specifically, the cooling air is supplied from the power feeding window 52a through the interior of the waveguide 52 and blown to the lamp 54. As another alternative, the cooling air is guided using a guide plate or the like and blown to the lamp 54 from outside the mesh plate 53b.
Operation of the conventional microwave-excited discharge lamp apparatus will be described.
Upon application of a high voltage to the microwave generator 51 from the high voltage power supply 55, the microwave is radiated from the antenna 51a of the microwave generator 51 into the waveguide 52. This microwave is propagated through the waveguide 52 and radiated to the cavity 53 from the power feeding window 52a formed in the waveguide 52. As a result, the predetermined microwave electromagnetic field is formed in the internal space of the cavity 53. The microwave electromagnetic field causes a dielectric breakdown of the rare gas and thus starts the plasma discharge. The plasma discharge increases the temperature of the inner wall of the lamp 54, whereby the mercury and the metal halide are vaporized, thereby increasing the internal pressure of the lamp 54. As long as the temperature at the coldest point of the inner wall and the internal pressure are stabilized at a predetermined value, respectively, i.e. in steady state operating condition, light having a predetermined emission spectrum is generated in the lamp 54 by the plasma discharge of the metal vapor. This light is radiated out through the mesh plate 53b from the cavity 53 as the light output. In the above-mentioned steady state operating condition, the pressure of the metal vapor represents a larger proportion of the internal pressure of the lamp 54 than that of the rare gas.
The above-mentioned conventional microwave-excited discharge lamp apparatus is required to comprise component members such as the pair of the nozzles and the air compressor used exclusively for cooling the lamp in accordance with the size of the lamp and the output of the magnetron. As a result, the configuration of the microwave-excited discharge lamp apparatus is complicated and becomes bulky. These problems present themselves conspicuously especially when the lamp is reduced in size for reducing the size of the light source. The reason is that in the case that the lamp is reduced in size, the plasma discharge occurs in the vicinity of the inner wall of the lamp, resulting in an increased tube wall temperature of the lamp. Thereby, the thermal degeneration of the lamp is accelerated, so that the lamp breaks or devitrifies, often shortening the life of the lamp. Further, with the increase in the temperature of the inner wall, the steady state operating condition of the lamp becomes unstable, thereby deteriorating the lighting characteristic. As a result, with the conventional microwave-excited discharge lamp apparatus having a smaller lamp, it is always necessary to provide the above-mentioned component members dedicated to cooling the lamp, thereby complicating and making bulky the configuration of the apparatus.
Furthermore, in the case that the light output from the light source is used by means of converging the light through a lens or a reflecting mirror, reducing the size of the light source is strongly demanded in order to efficiently extract the light output. As described in "Small Long-Lived Stable Light Source for Projection-Display Applications," International Symposium Digest, Technical Report, Vol. 24, pp. 716-719, for example, when the lamp is used as a backlight source for a projection display, it is strongly required that the lamp attains to the small size in order to efficiently extract the light output from the backlight source.
However, in the conventional microwave-excited discharge lamp apparatus, when the lamp attains to the small size, there are problems that the configuration of the apparatus complicates and makes bulky as described above. As a result, it has been difficult to use the conventional microwave-excited discharge lamp apparatus as the backlight source of the projection display or the like.
Further, with the conventional configuration in which the cooling air, after cooling the magnetron, is blown to the lamp from a power feeding window, the internal space of the cavity is suddenly expanded from the power feeding window. Therefore, the cooling air is excessively diffused when blowing into the internal space of the cavity from the power feeding window. Thereby, it was impossible to cool the lamp efficiently. Also, even with the conventional configuration in which the cooling air is blown from a mesh plate side, the mesh plate blocks the flow of the cooling air, and therefore the lamp cannot be cooled efficiently.